Vessel sealing system

ABSTRACT

An electrosurgical system is disclosed. The electrosurgical system includes an electrosurgical generator adapted to supply electrosurgical energy to tissue. The electrosurgical generator includes impedance sensing circuitry which measures impedance of tissue, a microprocessor configured to determine whether a tissue reaction has occurred as a function of a minimum impedance value and a predetermined rise in impedance, wherein tissue reaction corresponds to a boiling point of tissue fluid, and an electrosurgical instrument including at least one active electrode adapted to apply electrosurgical energy to tissue.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/652,932, entitled “Vessel Sealing System”, by Robert Wham etal., now U.S. Pat. No. 8,591,506, which is a continuation of U.S. patentapplication Ser. No. 12/057,557 entitled “Vessel Sealing System”, byRobert Wham et al., now U.S. Pat. No. 8,287,528, which is a continuationof U.S. patent application Ser. No. 10/626,390 also entitled “VesselSealing System”, by Robert Wham et al., now U.S. Pat. No. 7,364,577,which is a continuation-in-part application of U.S. patent applicationSer. No. 10/073,761 also entitled “Vessel Sealing System”, by RobertWham et al., now U.S. Pat. No. 6,796,981, which is acontinuation-in-part of U.S. patent application Ser. No. 09/408,944 alsoentitled “Vessel Sealing System”, by Robert Wham et al., now U.S. Pat.No. 6,398,779, and which claims priority to U.S. Provisional PatentApplication Ser. No. 60/105,417 filed on Oct. 23, 1998. The disclosureof each Patent Application is incorporated by reference herein in itsentirety.

FIELD

This invention relates generally to medical instruments and, inparticular, to generators that provide radio frequency (RF) energyuseful in sealing tissue and vessels during electrosurgical and otherprocedures. Electrosurgical generators are employed by surgeons to cutand coagulate the tissue of a patient.

BACKGROUND

High frequency electrical power, which may be also referred to as radiofrequency (RF) power or energy, is produced by the electrosurgicalgenerator and applied to the tissue by an electrosurgical tool. Bothmonopolar and bipolar configurations are commonly used duringelectrosurgical procedures.

Electrosurgical techniques can be used to seal small diameter bloodvessels and vascular bundles. Another application of electrosurgicaltechniques is in tissue fusion wherein two layers of tissue are graspedand clamped together by a suitable electrosurgical tool while theelectrosurgical RF energy is applied. The two layers of tissue are thenfused together.

At this point it is significant to note that the process of coagulatingsmall vessels is fundamentally different than vessel sealing or tissuefusion. For the purposes herein the term coagulation can be defined as aprocess of desiccating tissue wherein the tissue cells are ruptured anddried. Vessel sealing or tissue fusion can both be defined asdesiccating tissue by the process of liquefying the collagen in thetissue so that it crosslinks and reforms into a fused mass. Thus, thecoagulation of small vessels if generally sufficient to close them,however, larger vessels normally need to be sealed to assure permanentclosure.

However, and as employed herein, the term “electrosurgical desiccation”is intended to encompass any tissue desiccation procedure, includingelectrosurgical coagulation, desiccation, vessel sealing, and tissuefusion.

One of the problems that can arise from electrosurgical desiccation isundesirable tissue damage due to thermal effects, wherein otherwisehealthy tissue surrounding the tissue to which the electrosurgicalenergy is being applied is thermally damaged by an effect known in theart as “thermal spread”. During the occurrence of thermal spread excessheat from the operative site can be directly conducted to the adjacenttissue, and/or the release of steam from the tissue being treated at theoperative site can result in damage to the surrounding tissue.

It can be appreciated that it would be desirable to provide anelectrosurgical generator that limited the possibility of the occurrenceof thermal spread.

Another problem that can arise with conventional electrosurgicaltechniques is a buildup of eschar on the electrosurgical tool orinstrument. Eschar is a deposit that forms on working surface(s) of thetool, and results from tissue that is electrosurgically desiccated andthen charred. One result of the buildup of eschar is a reduction in theeffectiveness of the surgical tool. The buildup of eschar on theelectrosurgical tool can be reduced if less heat is developed at theoperative site.

It has been well established that a measurement of the electricalimpedance of tissue provides an indication of the state of desiccationof the tissue, and this observation has been utilized in someelectrosurgical generators to automatically terminate the generation ofelectrosurgical power based on a measurement of tissue impedance.

At least two techniques for determining an optimal amount of desiccationare known by those skilled in this art. One technique sets a thresholdimpedance, and terminates electrosurgical power when the measured tissueimpedance crosses the threshold. A second technique terminates thegeneration of electrosurgical power based on dynamic variations in thetissue impedance.

A discussion of the dynamic variations of tissue impedance can be foundin a publication entitled “Automatically Controlled BipolarElectrocoagulation”, Neurosurgical Review, 7:2-3, pp. 187-190, 1984, byVallfors and Bergdahl. FIG. 2 of this publication depicts the impedanceas a function of time during the heating of a tissue, and the authorsreported that the impedance value of tissue was observed to be near to aminimum value at the moment of coagulation. Based on this observation,the authors suggest a micro-computer technique for monitoring theminimum impedance and subsequently terminating the output power to avoidcharring the tissue.

Another publication by the same authors, “Studies on Coagulation and theDevelopment of an Automatic Computerized Bipolar Coagulator”, Journal ofNeurosurgery, 75:1, pp. 148-151, July 1991, discusses the impedancebehavior of tissue and its application to electrosurgical vesselsealing, and reports that the impedance has a minimum value at themoment of coagulation.

The following U.S. Patents are also of interest in this area. U.S. Pat.No. 5,540,684, Hassler, Jr. addresses the problem associated withturning off the RF energy output automatically after the tissueimpedance has fallen from a predetermined maximum, subsequently risenfrom a predetermined minimum and then reached a particular threshold. Astorage device records maximum and minimum impedance values, and acircuit determines the threshold. U.S. Pat. No. 5,472,443, Cordis etal., discusses a variation of tissue impedance with temperature, whereinthe impedance is shown to fall, and then to rise, as the temperature isincreased. FIG. 2 of this patent shows a relatively lower temperatureRegion A where salts contained in body fluids are believed todissociate, thereby decreasing the electrical impedance. A relativelynext higher temperature Region B is where the water in the tissue boilsaway, causing the impedance to rise. The next relatively highertemperature Region C is where the tissue becomes charred, which resultsin a slight lowering of the electrical impedance. U.S. Pat. No.4,191,188, Belt et al., discloses the use of two timers whose dutycycles are simultaneously and proportionately adjusted so that highfrequency signal bursts are constantly centered about the peak powerpoint, regardless of duty cycle variations.

Also of interest is U.S. Pat. No. 5,827,271, Buysse et al., “EnergyDelivery System for Vessel Sealing”, which employs a surgical toolcapable of grasping a tissue and applying an appropriate amount ofclosure force to the tissue, and for then conducting electrosurgicalenergy to the tissue concurrently with the application of the closureforce. FIG. 2 of this patent, shown herein as FIG. 1 for depicting theprior art, illustrates a set of power curves which represent theelectrosurgical power delivered to the tissue as a function of thetissue impedance. At low impedances, the electrosurgical power isincreased by rapidly increasing the output current. The increase inelectrosurgical power is terminated when a first impedance breakpoint,labeled as 1, is reached (e.g. <20 ohms). Next, the electrosurgicalpower is held approximately constant until proteins in the vessels andother tissues have melted. The impedance at which this segment endsvaries in accordance with the magnitude of the RMS power. For example,where the maximum RMS power is approximately 125 Watts, the segment (B)ends at about 128 ohms. When a lower power is used (e.g., 75 Watts), thesegment (C) may end at an impedance value of 256 ohms. Next, the outputpower is lowered to less than one half the maximum value, and the lowerpower delivery is terminated when a second impedance breakpoint isreached (2.048×103 ohms). Alternatives to using the impedance fordetermining the second breakpoint are the use of I-V phase angle, or themagnitude of the output current.

Based on the foregoing it should be evident that electrosurgery requiresthe controlled application of RF energy to an operative tissue site. Toachieve successful clinical results during surgery, the electrosurgicalgenerator should produce a controlled output RF signal having anamplitude and wave shape that is applied to the tissue withinpredetermined operating levels. However, problems can arise duringelectrosurgery when rapid desiccation of tissue occurs resulting inexcess RF levels being applied to the tissue. These excess levelsproduce less than desirable tissue effects, which can increase thermalspread, or can cause tissue charring and may shred and disintegratetissue. It would be desirable to provide a system with more controlledoutput to improve vessel sealing and reduce damage to surroundingtissue. The factors that affect vessel sealing include the surgicalinstrument utilized, as well as the generator for applying RF energy tothe instrument jaws. It has been recognized that the gap between theinstrument jaws and the pressure of the jaws against the tissue affecttissue sealing because of their impact on current flow. For example,insufficient pressure or an excessive gap will not supply sufficientenergy to the tissue and could result in an inadequate seal.

However, it has also been recognized that the application of RF energyalso affects the seal. For example, pulsing of RF energy will improvethe seal. This is because the tissue loses moisture as it desiccates andby stopping or significantly lowering the output the generator betweenpulses, this allows some moisture to return to the tissue for theapplication of next RF pulse. It has also been recognized by theinventors that varying each pulse dependent on certain parameters isalso advantageous in providing an improved seal. Thus, it would beadvantageous to provide a vessel sealing system which better controls RFenergy and which can be varied at the outset of the procedure toaccommodate different tissue structures, and which can further be variedduring the procedure itself to accommodate changes in the tissue as itdesiccates.

An accommodation for overvoltage clamping is also desirable. In thisregard, conventional overvoltage techniques use a means of clamping orclipping the excess overvoltage using avalanche devices such as diodes,zener diodes and transorbs so as to limit the operating levels. In thesetechniques the excess energy, as well as the forward conduction energy,is absorbed by the protection device and inefficiently dissipated in theform of heat. More advanced prior art techniques actively clamp only theexcess energy using a predetermined comparator reference value, butstill absorb and dissipate the excess energy in the form of heat.

U.S. Pat. No. 5,594,636 discloses a system for AC to AC power conversionusing switched commutation. This system addresses overvoltage conditionswhich occur during switched commutation by incorporating an activeoutput voltage sensing and clamping using an active clamp voltageregulator which energizes to limit the output. The active clamp switchesin a resistive load to dissipate the excess energy caused by theovervoltage condition.

Other patents in this area include U.S. Pat. No. 5,500,616, whichdiscloses an overvoltage clamp circuit, and U.S. Pat. No. 5,596,466,which discloses an isolated half-bridge power module. Both of thesepatents identify output overvoltage limiting for all power devices, andovervoltage limit protection is provided for power devices by usingproportionately scaled zeners to monitor and track the output offvoltage of each device to prevent power device failure. The zener deviceis circuit configured such that it provides feedback to the gate of thepower device. When zener avalanche occurs the power device partiallyturns on, absorbing the excess overvoltage energy in conjunction withthe connective load.

Reference can also be had to U.S. Pat. No. 4,646,222 for disclosing aninverter incorporating overvoltage clamping. Overvoltage clamping isprovided by using diode clamping devices referenced to DC power sources.The DC power sources provide a predetermined reference voltage to clampthe overvoltage condition, absorbing the excess energy through clampdiodes which dissipate the excess voltage in the form of heat.

It would be advantageous as to provide an electrosurgical generatorhaving improved overvoltage limit and transient energy suppression.

SUMMARY

The foregoing and other problems are overcome by methods and apparatusin accordance with embodiments disclosed herein.

An electrosurgical generator includes a controlling data processor thatexecutes software algorithms providing a number of new and usefulfeatures. These features preferably include the generation of an initialpulse, that is a low power pulse of RF energy that is used to sense atleast one electrical characteristic of the tissue prior to starting anelectrosurgical desiccation cycle, such as a tissue sealing cycle. Thesensed electrical characteristic is then used as an input into thedetermination of initial sealing parameters, thereby making the sealingprocedure adaptive to the characteristics of the tissue to be sealed.Another feature preferably provided measures the time required for thetissue to begin desiccating, preferably by observing an electricaltransient at the beginning of an RF energy pulse, to determine and/ormodify further seal parameters. Another preferable feature performs atissue temperature control function by adjusting the duty cycle of theRF energy pulses applied to the tissue, thereby avoiding the problemsthat can result from excessive tissue heating. A further preferablefeature controllably decreases the RF pulse voltage with each pulse ofRF energy so that as the tissue desiccates and shrinks (thereby reducingthe spacing between the surgical tool electrodes), arcing between theelectrodes is avoided, as is the tissue destruction that may result fromuncontrolled arcing. Preferably a Seal Intensity operator control isprovided that enables the operator to control the sealing of tissue byvarying parameters other than simply the RF power.

The system disclosed herein preferably further provides a unique methodfor overvoltage limiting and transient energy suppression. Anelectrosurgical system uses dynamic, real-time automatic detuning of theRF energy delivered to the tissue of interest. More specifically, thistechnique automatically limits excess output RF voltages by dynamicallychanging the tuning in a resonant source of RF electrosurgical energy,and by altering the shape of the RF source signal used to develop theoutput RF signal. The inventive technique limits the excess outputtransient RF energy by a resonant detuning of the generator. This occursin a manner which does not clip or significantly distort the generatedRF output signal used in a clinical environment for electrosurgicalapplications.

A method for electrosurgically sealing a tissue, in accordance with thisdisclosure, preferably includes the steps of (A) applying an initialpulse of RF energy to the tissue, the pulse having characteristicsselected so as not to appreciably heat the tissue; (B) measuring a valueof at least one electrical characteristic of the tissue in response tothe applied first pulse; (C) in accordance with the measured at leastone electrical characteristic, determining an initial set of pulseparameters for use during a first RF energy pulse that is applied to thetissue; and (D) varying the pulse parameters of subsequent RF energypulses individually in accordance with at least one characteristic of anelectrical transient that occurs at the beginning of each individualsubsequent RF energy pulse. The method terminates the generation ofsubsequent RF energy pulses based upon a reduction in the output voltageor upon a determination that the electrical transient is absent.

The at least one characteristic that controls the variation of the pulseparameters is preferably a width of the electrical transient that occursat the beginning of each subsequent RF energy pulse. The initial set ofpulse parameters include a magnitude of a starting power and a magnitudeof a starting voltage, and the pulse parameters that are varied includea pulse duty cycle and a pulse amplitude. Preferably, the subsequent RFenergy pulses are each reduced in amplitude by a controlled amount froma previous RF energy pulse, thereby compensating for a decrease in thespacing between the surgical tool electrodes due to desiccation of thetissue between the electrodes.

The step of determining an initial set of pulse parameters preferablyincludes a step of using the measured value of at least one electricalcharacteristic of the tissue to readout the initial set of pulseparameters from an entry in a lookup table.

The step of determining an initial set of pulse parameters may alsopreferably include a step of reading out the initial set of pulseparameters from an entry in one of a plurality of lookup tables, wherethe lookup table is selected either manually or automatically, based onthe electrosurgical instrument or tool that is being used.

The method also preferably includes a step of modifying predeterminedones of the pulse parameters in accordance with a control input from anoperator. The predetermined ones of the pulse parameters that aremodified include a pulse power, a pulse starting voltage level, a pulsevoltage decay scale factor, and a pulse dwell time.

Preferably a circuit is coupled to the output of the electrosurgicalgenerator for protecting the output against an overvoltage condition,and includes a suppressor that detunes a tuned resonant circuit at theoutput for reducing a magnitude of a voltage appearing at the output. Inaccordance with this aspect of the disclosure, the circuit has acapacitance network in parallel with an inductance that forms a portionof the output stage of the generator. A voltage actuated switch, such asa transorb, couples an additional capacitance across the network upon anoccurrence of an overvoltage condition, thereby detuning the resonantnetwork and reducing the magnitude of the voltage output.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description when read in conjunctionwith the attached Drawings, wherein:

FIG. 1A is a graph that plots output power versus tissue impedance (Z)in ohms, in accordance with the operation of a prior art electrosurgicalgenerator;

FIG. 1B is a graph that plots output power versus impedance in ohms, inaccordance with the operation of an electrosurgical generator that is anaspect of this disclosure;

FIG. 2 is a simplified block diagram of an electrosurgical system thatcan be used to practice the teachings of this disclosure;

FIG. 3 is a perspective view of one embodiment of a surgical instrumenthaving bipolar forceps that are suitable for practicing this disclosure;

FIG. 4 is an enlarged, perspective view of a distal end of the bipolarforceps shown in FIG. 3;

FIG. 5 is a perspective view of an embodiment of a surgical instrumenthaving forceps that are suitable for use in an endoscopic surgicalprocedure utilizing the electrosurgical system disclosed herein;

FIG. 6A is a simplified block diagram of a presently preferredembodiment of the power control circuit of the electrosurgical generatorof FIG. 2;

FIG. 6B depicts the organization of a seal parameter lookup table (LUT)shown in FIG. 6A;

FIGS. 7A and 7B illustrate a presently preferred electrosurgicalgenerator output waveform of RMS current vs. time for implementing atleast the first pulse of the pulsed operation mode that is an aspect ofthis disclosure;

FIG. 8 depicts a full set of electrosurgical RF pulses in accordancewith this disclosure, and illustrates the voltage, current and powercharacteristics of the pulses, as well as the response of the tissueimpedance to the applied RF pulses;

FIG. 9A illustrates a Seal Intensity control that forms a part of thisdisclosure, while FIGS. 9B and 9C show a preferred variation in certainparameters from the seal parameter LUT based on different Seal Intensitysettings;

FIG. 10 is a simplified block diagram of a circuit for achieving anovervoltage limiting and transient energy suppression energy function;

FIG. 11 is a waveform diagram illustrating the effect of the operationof the circuit in FIG. 10;

FIG. 12 is a logic flow diagram that illustrates a method in accordancewith the system disclosed herein;

FIG. 13 is a more detailed logic flow diagram that illustrates a methodin accordance with the system disclosed herein;

FIG. 14 is a chart illustrating a fixed number of pulses determined fromthe measured impedance and the RMS current pulse width;

FIG. 15 illustrates a Seal Intensity control that forms a part of thisdisclosure; and

FIG. 16 is a logic flow diagram that illustrates another method inaccordance with the system disclosed herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An electrosurgical system 1, which can be used to practice thisinvention, is shown in FIG. 2. The system 1 can be used for sealingvessels 3 and other tissues of a patient, including ducts, veins,arteries and vascular tissue. The system 1 includes an electro-surgicalgenerator 2 and a surgical tool, also referred to herein as a surgicalinstrument 4. The surgical instrument 4 is illustrated by way ofexample, and as will become apparent from the discussion below, otherinstruments can be utilized. The electrosurgical generator 2, which isof most interest to the teachings herein, includes severalinterconnected sub-units, including an RF drive circuit 2A, a powercontrol circuit 2B, a variable D.C. power supply 2C and an outputamplifier 2D. The surgical instrument 4 is electrically connected to theelectrosurgical generator 2 using a plug 5 for receiving controlledelectrosurgical power therefrom. The surgical instrument 4 has some typeof end effector member 6, such as a forceps or hemostat, capable ofgrasping and holding the vessels and tissues of the patient. The member6, also referred to simply as end effector 6, is assumed, in thisembodiment, to be capable of applying and maintaining a relativelyconstant level of pressure on the vessel 3.

The member 6 is provided in the form of bipolar electrosurgical forcepsusing two generally opposing electrodes disposed on inner opposingsurfaces of the member 6, and which are both electrically coupled to theoutput of the electrosurgical generator 2. During use, differentelectric potentials are applied to each electrode. In that tissue is anelectrical conductor, when the forceps are utilized to clamp or graspthe vessel 3 therebetween, the electrical energy output from theelectrosurgical generator 2 is transferred through the interveningtissue. Both open surgical procedures and endoscopic surgical procedurescan be performed with suitably adapted surgical instruments 4. It shouldalso be noted that the member 6 could be monopolar forceps that utilizeone active electrode, with the other (return) electrode or pad beingattached externally to the patient, or a combination of bipolar andmonopolar forceps.

By way of further explanation, FIG. 3 is a perspective view of oneembodiment of the surgical instrument 4 having a bipolar end effectorimplemented as forceps 6A while FIG. 4 is an enlarged, perspective viewof a distal end of the bipolar forceps 6A shown in FIG. 3.

Referring now to FIGS. 3 and 4, a bipolar surgical instrument 4 for usewith open surgical procedures includes a mechanical forceps 20 and anelectrode assembly 21. In the drawings and in the description whichfollows, the term “proximal”, as is traditional, refers to the end ofthe instrument 4 which is closer to the user, while the term “distal”refers to the end which is further from the user.

Mechanical forceps 20 includes first and second members 9 and 11 whicheach have an elongated shaft 12 and 14, respectively. Shafts 12 and 14each include a proximal end and a distal end. Each proximal end of eachshaft portion 12, 14 includes a handle member 16 and 18 attached theretoto allow a user to effect movement of the two shaft portions 12 and 14relative to one another. Extending from the distal end of each shaftportion 12 and 14 are end effectors 22 and 24, respectively. The endeffectors 22 and 24 are movable relative to one another in response tomovement of handle members 16 and 18. These end effectors members 6A canbe referred to collectively as bipolar forceps.

Preferably, shaft portions 12 and 14 are affixed to one another at apoint proximate the end effectors 22 and 24 about a pivot 25. As such,movement of the handles 16 and 18 imparts movement of the end effectors22 and 24 from an open position, wherein the end effectors 22 and 24 aredisposed in spaced relation relative to one another, to a clamping orclosed position, wherein the end effectors 22 and 24 cooperate to graspthe tubular vessel 3 therebetween. Either one or both of the endeffectors 22, 24 can be movable.

As is best seen in FIG. 4, end effector 24 includes an upper or firstjaw member 44 which has an inner facing surface and a plurality ofmechanical interfaces disposed thereon which are dimensioned toreleasable engage a portion of an electrode assembly 21, which may bedisposable. Preferably, the mechanical interfaces include sockets 41which are disposed at least partially through the inner facing surfaceof jaw member 44 and which are dimensioned to receive a complimentarydetent attached to an upper electrode 21A of the disposable electrodeassembly 21. The upper electrode 21A is disposed across from acorresponding lower electrode 21B. The end effector 22 includes a secondor lower jaw member 42 which has an inner facing surface which opposesthe inner facing surface of the first jaw member 44.

Preferably, shaft members 12 and 14 of the mechanical forceps 20 aredesigned to transmit a particular desired force to the opposing innerfacing surfaces of the jaw members 22 and 24 when clamped. Inparticular, since the shaft members 12 and 14 effectively act togetherin a spring-like manner (i.e., bending that behaves like a spring), thelength, width, height and deflection of the shaft members 12 and 14directly impacts the overall transmitted force imposed on opposing jawmembers 42 and 44. Preferably, jaw members 22 and 24 are more rigid thanthe shaft members 12 and 14 and the strain energy stored in the shaftmembers 12 and 14 provides a constant closure force between the jawmembers 42 and 44.

Each shaft member 12 and 14 also includes a ratchet portion 32 and 34.Preferably, each ratchet, e.g., 32, extends from the proximal end of itsrespective shaft member 12 towards the other ratchet 34 in a generallyvertically aligned manner such that the inner facing surfaces of eachratchet 32 and 34 abut one another when the end effectors 22 and 24 aremoved from the open position to the closed position. Each ratchet 32 and34 includes a plurality of flanges which project from the inner facingsurface of each ratchet 32 and 34 such that the ratchets 32 and 34 caninterlock in at least one position. In the embodiment shown in FIG. 3,the ratchets 32 and 34 interlock at several different positions.Preferably, each ratchet position holds a specific, i.e., constant,strain energy in the shaft members 12 and 14 which, in turn, transmits aspecific force to the end effectors 22 and 24 and, thus, to theelectrodes 21A and 21B. Also, preferably a stop is provided on one orboth of the end effectors 22, 24 to maintain a preferred gap between thejaws.

In some cases it may be preferable to include other mechanisms tocontrol and/or limit the movement of the jaw members 42 and 44 relativeto one another. For example, a ratchet and pawl system could be utilizedto segment the movement of the two handles into discrete units which, inturn, impart discrete movement to the jaw members 42 and 44 relative toone another.

FIG. 5 is a perspective view of an embodiment of the surgical instrument4 having end effector members or forceps 63 that are suitable for anendoscopic surgical procedure. The end effector member 63 is depicted assealing the tubular vessel 3 through a cannula assembly 48.

The surgical instrument 4 for use with endoscopic surgical proceduresincludes a drive rod assembly 50 which is coupled to a handle assembly54. The drive rod assembly 50 includes an elongated hollow shaft portion52 having a proximal end and a distal end. An end effector assembly 63is attached to the distal end of shaft 52 and includes a pair ofopposing jaw members. Preferably, handle assembly 54 is attached to theproximal end of shaft 52 and includes an activator 56 for impartingmovement of the forceps jaw members of end effector member 63 from anopen position, wherein the jaw members are disposed in spaced relationrelative to one another, to a clamping or closed position, wherein thejaw members cooperate to grasp tissue therebetween.

Activator 56 includes a movable handle 58 having an aperture 60 definedtherein for receiving at least one of the operator's fingers and a fixedhandle 62 having an aperture 64 defined therein for receiving anoperator's thumb. Movable handle 58 is selectively moveable from a firstposition relative to fixed handle 62 to a second position in the fixedhandle 62 to close the jaw members. Preferably, fixed handle 62 includesa channel 66 which extends proximally for receiving a ratchet 68 whichis coupled to movable handle 58. This structure allows for progressiveclosure of the end effector assembly, as well as a locking engagement ofthe opposing jaw members. In some cases it may be preferable to includeother mechanisms to control and/or limit the movement of handle 58relative to handle 62 such as, e.g., hydraulic, semi-hydraulic and/orgearing systems. As with instrument 4, a stop can also be provided tomaintain a preferred gap between the jaw members.

The handle 62 includes handle sections 62 a and 62 b, and is generallyhollow such that a cavity is formed therein for housing various internalcomponents. For example, the cavity can house a PC board which connectsthe electrosurgical energy being transmitted from the electrosurgicalgenerator 2 to each jaw member, via connector 5. More particularly,electrosurgical energy generated from the electrosurgical generator 2 istransmitted to the handle PC board by a cable 5A. The PC board divertsthe electrosurgical energy from the generator into two differentelectrical potentials which are transmitted to each jaw member by aseparate terminal clip. The handle 62 may also house circuitry thatcommunicates with the generator 2, for example, identifyingcharacteristics of the electrosurgical tool 4 for use by theelectrosurgical generator 2, where the electrosurgical generator 2 mayselect a particular seal parameter lookup table based on thosecharacteristics (as described below).

Preferably, a lost motion mechanism is positioned between each of thehandle sections 62 a and 62 b for maintaining a predetermined or maximumclamping force for sealing tissue between the jaw members.

Having thus described two exemplary and non-limiting embodiments ofsurgical instruments 4 that can be employed with the electrosurgicalgenerator 2, a description will now be provided of various aspects ofthe inventive electrosurgical generator 2.

FIG. 6A is a block diagram that illustrates the power control circuit 2Bof FIG. 2 in greater detail. The power control circuit 2B includes asuitably programmed data processor 70 that is preferably implemented asone or more microcontroller devices. In a most preferred embodimentthere are two principal microcontrollers, referred to as a mainmicrocontroller 70A and a feedback microcontroller 70B. These twomicrocontrollers are capable of communicating using shared data that isstored and retrieved from a shared read/write memory 72. A controlprogram for the data processor 70 is stored in a program memory 74, andincludes software routines and algorithms for controlling the overalloperation of the electrosurgical generator 2. In general, the feedbackmicrocontroller 70B has a digital output bus coupled to an input of adigital to analog converter (DAC) block 76 which outputs an analogsignal. This is a system control voltage (SCV), which is applied to thevariable DC power supply 2C to control the magnitude of the voltage andcurrent of output RF pulses.

An analog to digital converter (ADC) block 78 receives analog inputs andsources a digital input bus of the feedback microcontroller 70B. Usingthe ADC block 78 the microcontroller 70B is apprised of the value of theactual output voltage and the actual output current, thereby closing thefeedback loop with the SCV signal. The values of the output voltage andcurrent can be used for determining tissue impedance, power and energydelivery for the overall, general control of the applied RF energywaveform. It should be noted that at least the ADC block 78 can be aninternal block of the feedback microcontroller 70B, and need not be aseparate, external component. It should be further noted that the sameanalog signals can be digitized and read into the master microcontroller70A, thereby providing redundancy. The master microcontroller 70Acontrols the state (on/off) of the high voltage (e.g., 190V max) powersupply as a safety precaution, controls the front panel display(s), suchas a Seal Intensity display, described below and shown in FIG. 9A, andalso receives various input switch closures, such as a Seal Intensityselected by an operator.

It is noted that in a preferred embodiment of the electrosurgicalgenerator 2 a third (waveform) microcontroller 70C is employed togenerate the desired 470 kHz sinusoidal waveform that forms the basis ofthe RF pulses applied to the tissue to be sealed, such as the vessel 3(FIG. 2). The waveform microcontroller 70C is controlled by the feedbackmicrocontroller 70B and is programmed thereby. An output signal linefrom the feedback microcontroller 70B is coupled to a Reset input of thewaveform microcontroller 70C to essentially turn the waveformmicrocontroller 70C on and off to provide the pulsed RF signal inaccordance with an aspect of this disclosure. This particulararrangement is, of course, not to be viewed in a limiting sense upon thepractice of this system, as those skilled in the art may derive a numberof methods and circuits for generating the desired RF pulses inaccordance with the teachings found herein.

As an overview, the software algorithms executed by the data processor70 provide the following features. First, and referring now also to thepreferred waveform depicted in FIGS. 7A and 7B, a low power initialpulse of RF energy is used to sense at least one electricalcharacteristic of the tissue prior to starting the seal cycle. Second,the sensed electrical characteristic of the tissue is used as an inputinto the determination of the initial sealing parameters, thereby makingthe sealing procedure adaptive to the characteristics of the tissue tobe sealed. Third, the technique measures the time required for thetissue to begin desiccating, preferably by observing an electricaltransient, to determine and/or modify further seal parameters. Fourth,the technique performs a tissue temperature control function byadjusting the duty cycle of RF pulses applied to the tissue, therebyavoiding excessive tissue heating and the problems that arise fromexcessive tissue heating. This is preferably accomplished by using atleast one calculated seal parameter related to the time required for thetissue to begin desiccating. Fifth, the technique controllably changesthe RF pulse voltage with each pulse of RF energy DEL as the tissuedesiccates and shrinks (thereby reducing the pacing between the surgicalinstrument electrodes), arcing between the instrument electrodes (e.g.21A and 21B of FIG. 4) is avoided, as is the tissue destruction that mayresult from such uncontrolled arcing This is also preferablyaccomplished by using at least one calculated seal parameter that isrelated to the time required for the tissue to begin desiccating. Sixth,the above-mentioned Seal Intensity front panel control (FIG. 9A) enablesthe operator to control the sealing of tissue by varying parametersother than simply the RF power. These various aspects of this disclosureare now described in further detail.

Referring now also to the logic flow diagram of FIG. 13, the impedancesensing feature is implemented at the beginning of the seal cycle,wherein the electrosurgical generator 2 senses at least one electricalcharacteristic of the tissue, for example, impedance, I-V phaserotation, or the output current, by using a short burst of RF energy(FIG. 13, Steps A and B). The electrical characteristic of the tissuemay be measured at any frequency or power level, but preferably isperformed at the same frequency as the intended working frequency (e.g.,470 kHz). In a most preferred case the short burst of RF energy(preferably less than about 200 millisecond, and more preferably about100 millisecond) is a 470 kHz sine wave with approximately 5 W of power.The initial pulse RF power is made low, and the pulse time is made asshort as possible, to enable an initial tissue electrical characteristicmeasurement to be made without excessively heating the tissue.

In a most preferred embodiment the electrical characteristic sensed isthe tissue impedance which is employed to determine an initial set ofparameters that are input to the sealing algorithm, and which are usedto control the selection of sealing parameters, including the startingpower, current and voltage (FIG. 13, Step C). Other sealing parametersmay include duty cycle and pulse width. Generally, if the sensedimpedance is in the lower ranges, then the initial power and startingvoltage are made relatively lower, the assumption being that the tissuewill desiccate faster and require less energy. If the sensed impedanceis in the higher ranges, the initial power and starting voltage are maderelatively higher, the assumption being that the tissue will desiccateslower and require more energy.

In other embodiments at least one of any other tissue electricalcharacteristic, for example, the voltage or current, can be used to setthe parameters. These initial parameters are preferably modified inaccordance with the setting of the Seal Intensity control input (FIG.13, Step D), as will be described in further detail below.

Referring again to FIG. 13, Step C, the sensed impedance is employed todetermine which set of values are used from a seal parameter lookuptable (LUT) 80 (see FIGS. 6A and 6B). The seal parameter look up tablemay one of a plurality that are stored in the generator or accessible tothe generator. Furthermore, the seal parameter table may be selectedmanually or automatically, based on, for example, the electrosurgicaltool or instrument being employed. The specific values read from theseal parameter LUT 80 (FIG. 6B) are then adjusted based on the SealIntensity front panel setting 82 (FIG. 13, Step D), as is shown moreclearly in FIGS. 9A and 9B. In a preferred, but not limiting embodiment,the values read from the seal parameter LUT 80 comprise the power, themaximum voltage, starting voltage, minimum voltage, voltage decay,voltage ramp, maximum RF on time, maximum cool scale factor, pulseminimum, pulse dwell time, pulse off time, current and the desired pulsewidth. In a preferred, but not limiting embodiment, the seal parametervalues adjusted by the Seal Intensity front panel setting 82 (FIGS. 9Aand 9B) comprise the power, starting voltage, voltage decay, and pulsedwell time.

FIG. 1B is a graph that plots output power versus impedance in ohms forthe disclosed electrosurgical generator. The plot labeled “Intensity Bar1” shows the electrosurgical generator power output versus impedancewhen the “VLOW” setting 82A (FIG. 9A) of the Seal Intensity front panelsetting 82 is selected. The plot labeled Intensity Bar 2 shows the poweroutput of the electrosurgical generator when the “LOW” setting 82B ofthe Seal Intensity front panel setting 82 is selected. The plot labeledIntensity Bars 3, 4, 5, shows the power output of the electrosurgicalgenerator when the “MED” 82C, “HIGH” 82D or “VHIGH” 82E Seal Intensityfront panel settings 82 are selected. The Seal Intensity front panelsettings 82 adjust the seal parameter values as shown in FIG. 9B. Thesevalues may be adjusted depending on instrument used, tissuecharacteristics or surgical intent.

Discussing this aspect of the disclosure now in further detail, andreferring as well to FIGS. 7A, 7B and 8, the selected Seal ParameterTable, adjusted by the Seal Intensity front panel settings is thenutilized by the RF energy generation system and an initial RF sealingpulse is then started.

As each pulse of RF energy is applied to the tissue, the currentinitially rises to a maximum (Pulse Peak) and then, as the tissuedesiccates and the impedance rises due to loss of moisture in thetissue, the current falls. Reference in this regard can be had to thecircled areas designated as “A” in the I_(rms) waveform of FIG. 8. Theactual width of the resulting electrical transient, preferably a currenttransient “A”, is an important factor in determining what type andamount of tissue is between the jaws (electrodes) of the surgicalinstrument 4 (measured from “Full Power RF Start” to “Pulse Low andStable”.) The actual current transient or pulse width is also employedto determine the changes to, or the values of, the parameters of thepulse duty cycle (“Dwell Time”) and to the change of the pulse voltage,as well as other parameters. This parameter can also be used todetermine whether the tissue seal has been completed, or if the surgicalinstrument 4 has shorted.

As an alternative to directly measuring the pulse width, the rate ofchange of an electrical characteristic (for example current, voltage,impedance, etc.) of the transient “A” (shown in FIG. 7B) may be measuredperiodically (indicated by the reference number 90 shown in FIG. 7B)over the time the transient occurs. The rate of change of the electricalcharacteristic may be proportional to the width, At 95 of the transient“A”, defined by the relationship:Δt≈de/dt

where de/dt is the change in the electrical characteristic over time.This rate of change may then be used to provide an indication of thewidth of the transient “A” in determining the type and amount of tissuethat is between the jaws (electrodes) of the surgical instrument 4, aswell as the subsequent pulse duty cycle (“Dwell Time”), the amount ofsubsequent pulse voltage reduction, as well as other parameters.

Referring to FIG. 13, Step E, a subsequent RF energy pulse is applied tothe tissue, and the pulse width of the leading edge current transient ismeasured (FIG. 13, Step F). A determination is made if the currenttransient is present. If it is, control passes via connector “a” to StepH, otherwise control passes via connector “b” to Step K.

Assuming that the current transient is present, and referring to FIG.13, Step H, if the current transient pulse is wide, for example,approximately in the range of 500-1000 ms, then one can assume thepresence of a large amount of tissue, or tissue that requires more RFenergy to desiccate. Thus, the Dwell Time is increased, and an increaseor small reduction is made in the amplitude of the next RF pulse (seethe Vrms waveform in FIG. 8, and FIG. 13, Step I). If the currenttransient pulse is narrow, for example, about 250 ms or less (indicatingthat the tissue impedance rapidly rose), then one can assume a smallamount of tissue, or a tissue type that requires little RF energy todesiccate is present. Other ranges of current transient pulse widths canalso be used. The relationship between the current transient pulse widthand the tissue characteristics may be empirically derived. In this casethe Dwell Time can be made shorter, and a larger reduction in theamplitude of the next RF pulse can be made as well (FIG. 13, Step J).

If a current pulse is not observed at FIG. 13, Step G, it may be assumedthat either the instrument 4 has shorted, the tissue has not yet begunto desiccate, or that the tissue has been fully desiccated and, thus,the seal cycle is complete. The determination of which of the above hasoccurred is preferably made by observing the tissue impedance at FIG.13, Steps K and M. If the impedance is less than a low threshold value(THRESH_(L)), then a shorted instrument 4 is assumed (FIG. 13, Step L),while if the impedance is greater than a high threshold value(THRESH_(H)), then a complete tissue seal is assumed (FIG. 13, Step N).

If the tissue impedance is otherwise found to be between the high andlow threshold values, a determination is made as to whether the Max RFOn Time has been exceeded. If the Max RF On Time has been exceeded, itis assumed that the seal cannot be successfully completed for somereason and the sealing procedure is terminated. If the Max RF On Timehas not been exceeded then it is assumed that the tissue has not yetreceived enough RF energy to start desiccation, and the seal cyclecontinues (connector “c”).

After the actual pulse width measurement has been completed, the DwellTime is determined based on the actual pulse width and on the Dwell Timefield in the seal parameter LUT 80 (see FIG. 6B.) The RF pulse iscontinued until the Dwell Time has elapsed, effectively determining thetotal time that RF energy is delivered for that pulse. The RF pulse isthen turned off or reduced to a very low level for an amount of timespecified by the Pulse Off field. This low level allows some moisture toreturn to the tissue.

Based on the initial Desired Pulse Width field of the seal parameter LUT80 for the first pulse, or, for subsequent pulses, the actual pulsewidth of the previous pulse, the desired voltage limit kept constant oradjusted based on the Voltage Decay and Voltage Ramp fields. The desiredvoltage limit is kept constant or raised during the pulse if the actualpulse width is greater than the Desired Pulse Width field (or lastactual) pulse width), and is kept constant or lowered if the actualpulse width is less than the Desired Pulse Width field (or the lastactual pulse width).

When the Desired Voltage has been reduced to the Minimum Voltage field,then the RF energy pulsing is terminated and the electrosurgicalgenerator 2 enters a cool-down period having a duration that is set bythe Maximum Cool SF field and the actual pulse width of the first pulse.

Several of the foregoing and other terms are defined with greaterspecificity as follows (see also FIGS. 7A and 7B).

The Actual Pulse width is the time from pulse start to pulse low. ThePulse Peak is the point where the current reaches a maximum value, anddoes not exceed this value for some predetermined period of time(measured in milliseconds). The peak value of the Pulse Peak can bereached until the Pulse Peak-X % value is reached, which is the pointwhere the current has decreased to some predetermined determinedpercentage, X, of the value of Pulse Peak. Pulse Low is the point wherethe current reaches a low point, and does not go lower for anotherpredetermined period of time. The value of the Maximum RF On Time or MAXPulse Time is preferably preprogrammed to some value that cannot bereadily changed. The RF pulse is terminated automatically if the PulsePeak is reached but the Pulse Peak-X % value is not obtained with theduration set by the Maximum RF On Time field of the seal parameter LUT80.

Referring to FIG. 6B, the seal parameter LUT 80 is employed by thefeedback microcontroller 70B in determining how to set the variousoutputs that impact the RF output of the electrosurgical generator 2.The seal parameter LUT 80 is partitioned into a plurality of storageregions, each being associated with a particular measured initialimpedance. More particularly, the Impedance Range defines a plurality ofimpedance breakpoints (in ohms) which are employed to determine whichset of variables are to be used for a particular sealing cycle. Theparticular Impedance Range that is selected is based on the abovedescribed Impedance Sense State (FIGS. 7A and 7B) that is executed atthe start of the seal cycle. The individual data fields of the sealparameter LUT 80 are defined as follows.

The actual values for the Impedance Ranges of Low, Med Low, Med High, orHigh, are preferably contained in one of a plurality of tables stored inthe generator 2, or otherwise accessible to the generator 2. A specifictable may be selected automatically, for example, based on signalsreceived from the electrosurgical tool 4 being used, or by the operatorindicating what electrosurgical tool is in use.

Power is the RF power setting to be used (in Watts). Max Voltage is thegreatest value that the output voltage can achieve (e.g., range 0-about190V). Start Voltage is the greatest value that the first pulse voltagecan achieve (e.g., range 0-about 190V). Subsequent pulse voltage valuesare typically modified downwards from this value. The Minimum Voltage isthe voltage endpoint, and the seal cycle can be assumed to be completewhen the RF pulse voltage has been reduced to this value. The VoltageDecay scale factor is the rate (in volts) at which the desired voltageis lowered if the current Actual Pulse Width is less than the DesiredPulse Width. The Voltage Ramp scale factor is the rate at which thedesired voltage will be increased if the Actual Pulse Width is greaterthan the Desired Pulse Width. The Maximum RF On Time is the maximumamount of time (e.g., about 5-20 seconds) that the RF power can bedelivered, as described above. The Maximum Cool Down Time determines thegenerator cool down time, also as described above. Pulse Minimumestablishes the minimum Desired Pulse Width value. It can be noted thatfor each RF pulse, the Desired Pulse Width is equal to the Actual PulseWidth from the previous pulse, or the Desired Pulse field if the firstpulse. The Dwell Time scale factor was also discussed previously, and isthe time (in milliseconds) that the RF pulse is continued after thecurrent drops to the Pulse Low and Stable point (see FIGS. 7A and 7B).Pulse Off is the off time (in milliseconds) between RF pulses. DesiredPulse Width is a targeted pulse width and determines when the DesiredVoltage (Vset) is raised, lowered or kept constant. If the Actual PulseWidth is less than the Desired Pulse Width, then Vset is decreased,while if the Actual Pulse Width is greater than the Desired Pulse Width,then Vset is increased. If the Actual Pulse Width is equal to theDesired Pulse Width, then Vset is kept constant. The Desired Pulse Widthis used as the Desired Pulse Width for each sequential pulse. Ingeneral, a new Desired Pulse Width cannot be greater than a previousDesired Pulse Width, and cannot be less than Pulse Minimum.

By applying the series of RF pulses to the tissue, the surgicalgenerator 2 effectively raises the tissue temperature to a certainlevel, and then maintains the temperature relatively constant. If the RFpulse width is too long, then the tissue may be excessively heated andmay stick to the electrodes 21A, 21B of the surgical instrument 4,and/or an explosive vaporization of tissue fluid may damage the tissue,such as the vessel 3. If the RF pulse width is too narrow, then thetissue will not reach a temperature that is high enough to properlyseal. As such, it can be appreciated that a proper balance of duty cycleto tissue type is important.

During the pulse off cycle that is made possible in accordance with theteachings herein, the tissue relaxes, thereby allowing the steam to exitwithout tissue destruction. The tissue responds by rehydrating, which inturn lowers the tissue impedance. The lower impedance allows thedelivery of more current in the next pulse. This type of pulsedoperation thus tends to regulate the tissue temperature so that thetemperature does not rise to an undesirable level, while stillperforming the desired electrosurgical procedure, and may also allowmore energy to be delivered, and thus achieving better desiccation.

As each RF pulse is delivered to the tissue, the tissue desiccates andshrinks due to pressure being applied by the jaws of the surgicalinstrument 4. The inventors have realized that if the voltage applied tothe tissue is not reduced, then as the spacing between the jaws of thesurgical instrument 4 is gradually reduced due to shrinking of thetissue, an undesirable arcing can develop which may vaporize the tissue,resulting in bleeding.

As is made evident in the V_(RMS) trace of FIG. 8, and as was describedabove, the voltage of each successive RF pulse can be controllablydecreased, thereby compensating for the desiccation-induced narrowing ofthe gap between the surgical instrument electrodes 21A and 21B. That is,the difference in electric potential between the electrodes is decreasedas the gap between the electrodes decreases, thereby avoiding arcing.

As was noted previously, the Seal Intensity front panel adjustment isnot a simple RF power control. The adjustment of the seal intensity isaccomplished by adjusting the power of the electrosurgical generator 2,as well as the generator voltage, the duty cycle of the RF pulses, thelength of time of the seal cycle (e.g., number of RF pulses), and therate of voltage reduction for successive RF pulses. FIGS. 9B and 9Cillustrate an exemplary set of parameters (Power, Start Voltage, VoltageDecay and Dwell Time), and how they modify the contents of the sealparameter LUT 80 depending on the setting of the Seal Intensity control82 shown in FIG. 9A. Generally, higher settings of the Seal Intensitycontrol 82 increase the seal time and the energy delivered while lowersettings decrease the seal time and the energy delivered.

In the FIG. 9B embodiment, it is instinctive to note that for theMedium, High and Very High Seal Intensity settings the RF Power remainsunchanged, while variations are made instead in the Start Voltage,Voltage Decay and Dwell Time parameters.

Based on the foregoing it can be appreciated that an aspect of thisdisclosure is a method for electrosurgically sealing a tissue. Referringto FIG. 12, the method includes steps of: (A) applying an initial pulseof RF energy to the tissue, the pulse having characteristics selected soas not to excessively heat the tissue; (B) measuring at least oneelectrical characteristic of the tissue in response to the appliedpulse; (C) in accordance with the measured electrical characteristic,determining an initial set of pulse parameters for use during a first RFenergy pulse that is applied to the tissue; and (D) varying the pulseparameters of individual ones of subsequent RF energy pulses inaccordance with at least one characteristic of an electric currenttransient that occurs at the beginning of each individual one (pulses)of the subsequent RF energy pulses. The method can terminate thegeneration of subsequent RF energy pulses upon a determination that thecurrent transient is absent or that the voltage has been reduced to apredefined level. In another embodiment of the present invention, theinitial pulse may be combined with at least the first subsequent pulse.

Reference is now made to FIGS. 10 and 11 for a description of a novelovervoltage limit and transient energy suppression aspect of the systemdisclosed herein.

A bi-directional transorb TS1 normally is non-operational. As long asthe operating RF output levels stay below the turn-on threshold of TS1,electro surgical energy is provided at a controlled rate of tissuedesiccation. However, in the event that rapid tissue desiccation occurs,or that arcing is present in the surgical tissue field, the RF outputmay exhibit operating voltage levels in excess of the normal RF levelsused to achieve the controlled rate of tissue desiccation. If the excessvoltage present is left unrestrained, the tissue 3 may begin to exhibitundesirable clinical effects contrary to the desired clinical outcome.The TS1 is a strategic threshold that is set to turn on above normaloperating levels, but below and just prior to the RF output reaching anexcess voltage level where undesirable tissue effects begin to occur.The voltage applied across TS1 is proportionately scaled to follow theRF output voltage delivered to the tissue 3. The transorb TS1 isselected such that its turn on response is faster than the generatorsource RF signal. This allows the transorb TS1 to automatically trackand respond quickly in the first cycle of an excess RF outputovervoltage condition.

Note should be made in FIG. 10 of the capacitor components or networkC2, C3, and C4 that parallel the magnetic drive network (MDN1) which hasan inductive characteristic and is contained within the electrosurgicalgenerator 2. The combination of the inductive MDN1 and the capacitivenetworks forms a resonant tuned network which yields the waveshapeconfiguration of the RF source signal shown in FIG. 11.

A turn on of transorb device TS1, which functions as a voltagecontrolled switch, instantaneously connects the serial capacitance C1across the capacitor network C2, C3, and C4. An immediate change thenappears in the tuning of the resonant network mentioned above, whichthen instantaneously alters the waveshape of the RF source signal shownin FIG. 11. The time base T1 of the nominally half-sine signal shownincreases incrementally in width out to time T2, which automaticallylowers the peak voltage of the RF output signal. The peak voltagedecreases because the Voltage-Time product of the signal shown in FIG.11 is constant for a given operating quiescence. The concept of aVoltage-Time product is well known to those skilled in the art, and isnot further discussed herein.

As the peak voltage decreases, the excess overvoltage is automaticallylimited and is restricted to operating levels below that which causenegative clinical effects. Once the excess RF output voltage level fallsbelow the transorb threshold, the TS1 device turns off and theelectrosurgical generator 2 returns to a controlled rate of tissuedesiccation.

In the event that arcing is present in the surgical tissue field,undesirable excess transient RF energy may exist and may be reflected inthe RF output of the electrosurgical generator 2. This in turn maygenerate a corresponding excess RF output voltage that createssufficient transient overvoltage to turn on the transorb TS1. In thiscondition the cycle repeats as described above, where TS1 turns on,alters the resonant tuned network comprised of the magnetic andcapacitive components, and thus also alters the RF source signalwaveshape. This automatically reduces the excess overvoltage.

In accordance with this aspect of the disclosure, the excess RFtransient energy is suppressed and the overvoltage is limited by thedynamic, real-time automatic detuning of the RF energy delivered to thetissue being treated.

It should be noted that the embodiment of FIGS. 10 and 11 can be used toimprove the operation of conventional electrosurgical generators, aswell as with the novel pulsed output electrosurgical generator 2 thatwas described previously.

In an additional embodiment the measured electrical characteristic ofthe tissue, preferably the impedance (Z_(i)), and the RMS current pulsewidth (P_(W)) may be used to determine a fixed voltage reduction factor(V_(dec)) to be used for subsequent pulses, and to determine a fixednumber of pulses (P_(F)) to be delivered for the sealing procedure. Therelationship among the voltage reduction factor, the measured impedanceand the RMS current pulse width may be defined as V_(dec)=F (Z_(I),P_(W)), and the relationship among the number of pulses, the measuredimpedance and the RMS current pulse width may be defined as P_(F)=F′(Z₁, P_(W)). In FIG. 14 a fixed number of pulses, P_(F), 100 determinedfrom the measured impedance and the RMS current pulse width are shown.Each subsequent pulse may be reduced by the fixed voltage reductionfactor (V_(dec)) 110, also determined from the measured impedance andthe RMS current pulse width.

In a further additional embodiment, tissue sealing is accomplished bythe electrosurgical system described above by continuously monitoring orsensing the current or tissue impedance rate of change. If the rate ofchange increases above a predetermined limit, then RF pulsing isautomatically terminated by controlling the electrosurgical generator 2accordingly and any previously changed pulse parameters (e.g., power,voltage and current increments) are reset to the original defaultvalues. In this embodiment, the ending current or tissue impedance,i.e., the current or tissue impedance at the end of each RF pulse, isalso continuously monitored or sensed. The ending values are then usedto determine the pulse parameters for the subsequent RF pulse; todetermine if the seal cycle should end (based on the ending values ofthe last few RF pulses which did not change by more than a predeterminedamount); and to determine the duty cycle of the subsequent RF pulse.

Further, in this embodiment, RF power, pulse width, current and/orvoltage levels of subsequent RF pulses can be kept constant or modifiedon a pulse-by-pulse basis depending on whether the tissue has respondedto the previously applied RF energy or pulse (i.e., if the tissueimpedance has begun to rise). For example, if the tissue has notresponded to a previously applied RF pulse, the RF power output, pulsewidth, current and/or voltage levels are increased for the subsequent RFpulse.

Hence, since these RF pulse parameters can subsequently be modifiedfollowing the initial RF pulse, the initial set of RF pulse parameters,i.e., a magnitude of a starting RF power level, a magnitude of astarting voltage level, a magnitude of the starting pulse width, and amagnitude of a starting current level, are selected accordingly suchthat the first or initial RF pulse does not excessively heat the tissue.One or more of these starting levels are modified during subsequent RFpulses to account for varying tissue properties, if the tissue has notresponded to the previously applied RF pulse which includes the initialRF pulse.

The above functions are implemented by a seal intensity algorithmrepresented as a set of programmable instructions configured for beingexecuted by at least one processing unit of a vessel sealing system. Thevessel sealing system includes a Seal Intensity control panel formanually adjusting the starting voltage level, in a similar fashion asdescribed above with reference to FIGS. 9A and 9B.

As shown in FIG. 15, a preferred Seal Intensity control panel of thepresent inventive embodiment includes six settings, i.e., “Off” 150A,“VLOW” 150B, “LOW” 150C, “MED” 150D, “HIGH” 150E and “VHIGH” 150F. TheSeal Intensity front panel settings 150 adjust the seal parameter valuesof the Seal Parameter Table as shown by FIGS. 9B and 9C. The selectedSeal Parameter Table, adjusted by the Seal Intensity front panelsettings 150 is then utilized by an RF generation system, as describedabove, and an initial RF sealing pulse is then started.

The Seal Intensity front panel settings, as shown in FIGS. 9B and 9C,represent approximate parametric values of several preferredembodiments, identified as an example to achieve vessel sealingperformance in clinical procedures. The variety of tissue types andsurgical procedures requires the use of one or more Seal Intensity frontpanel settings.

FIG. 16 is a logic flow diagram that illustrates a method in accordancewith the vessel sealing system. At step A′, a RF pulse is applied totissue. At step B′, the current or tissue impedance rate of change iscontinuously monitored. At step C′, a determination is made whether thetissue impedance rate of change has passed a predetermined limit. Ifyes, at step D′, RF pulsing is terminated and any previously changedpulse parameters are reset back to the original defaults. If no, theprocess proceeds to step E′.

At step E′, a determination is made as to whether the RF pulse hasended. If no, the process loops back to step B′. If yes, the processproceeds to step F′. At step F′, the ending current or tissue impedanceis measured. At step G′, the measured ending values are used fordetermining if the seal cycle should end (based on the current level orending impedance of the last few RF pulses which did not change by morethan a predetermined amount). If yes, the process terminates at step H′.If no, the process continues at step I′, where the ending values areused for determining the pulse parameters, i.e., the power, pulse width,current and/or voltage levels, and the duty cycle of the subsequent RFpulse from an entry in one of a plurality of lookup tables. The processthen loops back to step A′. One of the plurality of lookup tables isselected manually or automatically, based on a choice of anelectrosurgical tool or instrument.

While the system has been particularly shown and described with respectto preferred embodiments thereof, it will be understood by those skilledin the art that changes in form and details may be made therein withoutdeparting from its scope and spirit.

What is claimed is:
 1. An electrosurgical generator, comprising: an RFenergy source configured to provide RF energy; an output configured tocouple the RF energy source to opposing first and second electrodes of asurgical instrument and provide the RF energy thereto, the opposingfirst and second electrodes when positioned about tissue forming a gaptherebetween; and a controller configured to regulate the RF energy fromthe RF energy source, the controller configured to decrease the outputof the RF energy as the gap between the first electrode and secondelectrode is decreased.
 2. The generator according to claim 1, whereinthe controller controllably decreases an amplitude of the RF energy tocompensate for a decrease in the gap due to thermally-induced shrinkageof tissue.
 3. The generator according to claim 1, wherein the controllerapplies a plurality of pulses of RF energy between the first and secondelectrodes and controllably decreases an amplitude between eachindividual RF energy pulse as the gap between the first and secondelectrodes decreases.
 4. The generator according to claim 3, wherein theamplitude is controllably decreased to compensate for a decrease in gapbetween the first and second electrodes due to thermally-inducedshrinkage of tissue.
 5. An electrosurgical generator comprising: acontroller configured to apply an initial pulse of RF energy to opposingfirst and second electrodes having a gap defined therebetween, theinitial pulse of RF energy forming an electrical potential between thefirst and second electrodes when positioned about tissue, the controllerconfigured to apply at least one subsequent pulse of RF energy to thefirst and second electrodes forming a subsequent electrical potentialbetween the first and second electrodes when positioned about tissue,the difference in the at least one subsequent pulse of RF energy andsubsequent electrical potential between first and second electrodesbeing decreased as the gap between the first and second electrodes isdecreased.
 6. An electrosurgical generator comprising: a controllerconfigured to apply an initial pulse of RF energy to the tissue, theinitial pulse forming an electrical potential between first and secondelectrodes positioned about tissue having a gap defined therebetween,the controller configured to decrease the amplitude of subsequent pulsesof RF energy as the gap between the first and second electrodes isdecreased.
 7. The generator according to claim 6 wherein the controllercontrollably decreases the amplitude of subsequent pulses of RF energyto compensate for a decrease in the gap between the first and secondelectrodes due to thermally-induced shrinkage of tissue.